The overall objective of this proposal is to analyze retroviral insertional mutants in transgenic mice which exhibit specific developmental abnormalities. The transgenic mice under study were derived from retrovirally infected embryonic stem cells (ES cells) using the ES cell-chimeric mouse system. Preliminary studies have identified one recessive embryonic lethal mutation due to retroviral insertion, in which abnormal development of certain embryonic cell lineages occurs. The retrovirus responsible for the mutation will be used as a molecular tag for cloning the mutated endogenous gene. The wild-type allele of this candidate developmental gene will be fully characterized, and the developmental actions and interactions of its gene product defined. To complement these studies, additional recessive mutations caused by retroviral insertion will be screened for an analyzed. One animal with abnormal limb development has already been found. Genetic tests to show that this phenotype is both heritable, and caused by a specific retroviral insertion will be carried out. Crosses between animals heterozygous for other proviruses have revealed the potential existence of additional embryonic lethal mutations, and the involvement of specific retroviruses will now be investigated. In addition, the potential mutagenic effect of the remaining retroviral integrations carried in one transgenic strain will be assessed. In summary, it is hoped that retroviral insertional mutagenesis and ES cell mediated transgenesis, by identifying and characterizing candidate developmental genes, will contribute to our understanding of specific developmental mechanisms.